(i) Technical Field
The present invention relates to a semiconductor device, and more particularly, to a semiconductor device having a GaN-based semiconductor layer.
(ii) Related Art
A semiconductor device using a compound semiconductor layer containing Ga (gallium) and N (nitrogen) (GaN-based semiconductor) has attracted attention as an amplifying device capable of operating at high frequencies and outputting high power. The GaN-based semiconductor is a semiconductor that contains gallium nitride (GaN), and is, for example, GaN, AlGaN that is a mixed crystal of GaN and aluminum nitride (AlN), InGaN that is a mixed crystal of GaN and indium nitride (InN), and AlInGaN that is a mixed crystal of GaN, AlN and InN.
In a case where a GaN-based semiconductor layer is grown on a silicon substrate, an underlying buffer layer is preferably formed in order to reduce stress resulting from the difference in crystal structure between the silicon substrate and the GaN-based semiconductor layer. The buffer layer may have a multilayer structure in which an AlN layer and a GaN layer are alternately stacked (see Japanese Patent Application Publication No. 2005-158889). There is known another buffer layer having a first AlxGa1-xN layer having an aluminum composition x as low as 0.1 and a second AlxGa1-xN layer having an aluminum composition x as high as 0.75, which different layers are alternately stacked (see Japanese Patent Application Publication No. 2007-067077). In the following, the aluminum composition x is the value of x in AlxGa1-xN.